The present invention relates to a thermal protection system, and more particularly to a reusable thermal protection system for a portion of the Space Shuttle Orbiter.
Space vehicles are subjected to increases in temperature during ascent and reentry. A heat shield is used to protect the metallic structure of the vehicle during such periods. During the early space programs, manned vehicles were used for single trips and the heat shields were formed of ablative material. However, with the advent of the Space Transportation System which used a Space Shuttle Orbiter designed to make numerous trips into space, it was necessary to develop a reusable thermal protection system. The Space Shuttle Orbiter is in some respects similar to a large airplane and consequently various areas of the vehicle are subject to varying surface temperature extremes. For example, the nose cone and leading edges of the wings are subject to the greatest heating and require a system which will provide protection for surface temperatures up to 2800.degree. F. (1811.degree. K.). A coated reinforced carbon-carbon composite material (RCC) has been found satisfactory for these areas. On the other hand, the cargo bay doors are subject to the least heating and only require a system which will provide protection for surface temperatures up to 700.degree. F. (644.degree. K.). It has been found that the flexible reusable surface insulation (FRSI) fully described in U.S. Pat. No. 4,151,800 is satisfactory for this area. The undersides of the wings and fuselage require a system which will provide protection for surface temperatures up to 2300.degree. F. (1533.degree. K.) while the sides of the fuselage only require protection for surface temperatures up to 1200.degree. F. (922.degree. K.). The high temperature system is known as high temperature resuable surface insulation (HRSI) and the low temperature system as low temperature reusable surface insulation (LRSI). Presently, ceramic tiles cut from rigidized blocks of highly refined, chemically pure, silica fibers are used for such systems. The material, known as LI-900, is manufactured by Lockheed Missle and Space Corporation. The tiles are attached to the orbiter by the attachment arrangement fully described in U.S. Pat. No. 4,124,733.
For areas around openings and other locations requiring greater mechanical strength, a denser, stronger silica material described in U.S. Pat. No. 3,952,082, and known as LI-2200, is used. In addition, a new fibrous refactory composite insulation material, described in U.S. Pat. No. 4,148,962, and known as FRCI, is being considered for use. The tiles of all three materials are coated with a reaction cured borosilicate glass coating made in accordance with U.S. Pat. No. 4,093,771. Further, the tiles are provided with a deposition of a silane to provide water repellency. However, the coating as well as the tile material itself is fragile and has a low impact resistance, and, therefore, is easily subject to damage. Also, the silane burns out, therefore the tile system must be refurbished between flights to repair damage and provide new water repellency.
An effort has been made to find other materials which have stronger surface properties, and require a minimum of refurbishment between flights. One approach is to use high temperature resistant resilient material, such as advanced carbon-carbon, as outer panels for the large high temperature area on the underside of the wings and fuselage. Since the thermal protection system of the Space Shuttle Orbiter experiences stresses from a wide variety of loading conditions, such as mechanical, aerodynamic, and vibro-acoustic, as well as extreme temperature variations, it is necessary that the panels used for thermal protection be of sufficient strength and attached to the substructure in such a manner as to be able to withstand the stresses and loads of space flight and that, at the same time, the system provide the required thermal properties without a weight penalty.
Consequently, an object of the present invention is to develop a thermal protection system of protective panels for the surfaces of the Space Shuttle Orbiter subject to high temperature, which will withstand the stresses and loads of repetitive space flights with a minimum of refurbishment.
Another object is to provide improved attaching means for a thermal protection system of protective panels in which the attachment means are insulated to minimize oxidation from heating.
A further object is to provide a thermal protection system of hexagonal protective panels.
Still another object is to improve the impact resistance and durability of the thermal protection system.